Corrosion of steel in concrete, especially reinforcement bars in concrete, has evolved over the past two decades to become the single most costly problem of its type in the U.S. Cost estimates for repairing the present damage on the highway bridges approach 20 billion dollars according to the Transportation Research Board. Cathodic protection systems are presently being used to mitigate corrosion in conventionally reinforced concrete members, and their use is being contemplated on prestressed bridge components. The use of embedded reference electrodes or reference cells in conjunction with cathodic protection of conventionally reinforced concrete bridge members is desirable, and is essential for application of cathodic protection to prestressed concrete bridge components.
For cathodic protection systems to be successful in mitigating corrosion without causing hydrogen embrittlement of the embedded steel reinforcement bars, the amount of polarization of the embedded steel must be very accurately controlled. To monitor accurately and to control the amount of polarization of prestressing steel, stable and reliable embedded reference electrodes are needed. Such reference electrodes can also prove useful during cathodic protection of conventionally reinforced bridge components to provide a means for cost-effective monitoring and to avoid overprotection.
Several different types of reference cells have been used in field trials and in full scale cathodic protection systems. Their field performance history along with the results of laboratory studies have indicated potential problems of long term stability and reliability. Although portable copper-copper sulfate (Cu--CuSO.sub.4) reference cells are widely used for surface potential surveys in which their performance has been satisfactory, the use of embedded Cu--Cu--SO.sub.4 reference cells has been restricted due to leakage, drying out and/or freezing of the electrolyte in the cell. Also, copper sulfate leaking from a cell can react with water in pores in the concrete and generate undesirable products.
The most widely used embedded reference cells in cathodic protection work are silver-silver chloride (Ag--AgCI), but due to the instability of these cells they are most often used only for short term monitoring of potentials rather than for cathodic protection systems. Their low temperature response is very erratic, and they are also sensitive to the chloride content of the concrete. Another drawback of these reference cells is that they develop a variable and high resistance. We have found the resistance between the Ag--AgCl cell and the reinforcement bar to vary from a few hundred to a few hundred thousand ohms for cells embedded in similar concrete in the same bridge deck and at the same temperature.
Zinc-zinc sulfate (Zn--ZnSO.sub.4) cells have been used extensively, but the stability of these cells is a problem. They are found to be sufficiently stable to conduct short-term monitoring and performed well in laboratory studies, but many of the Zn-ZnSO.sub.4 cells become unstable after one year of outdoor exposure and as a consequence, potential measurements can fluctuate widely. Zn--ZnSO.sub.4 cells are also strongly affected by low temperatures.
We have found the performance and stability of graphite that is used as a reference electrode can be significantly improved by selective treatment to improve exchange current densities when in equilibrium with air or oxygen and decrease peroxide accumulation. The forming of a stable carbon reference electrode by 1) hydrogen peroxide (H.sub.2 O.sub.2) chemical pre-treatment, 2) doping with manganese dioxide (MnO.sub.2), and 3) subsequent wetproofing with a thin porous fluorocarbon layer combined with a conductive ceramic outer layer has been discovered to give enhanced carbon reference electrode stability.
Kroon et al., in patent U.S. Pat. No. 4,255,241 entitled "Cathodic protection apparatus and method for steel reinforced concrete structures," describe a system comprising anodes encased in a carbonaceous material matrix and placed within slots cut in a concrete structure. A current for cathodic protection is applied between the anodes and the steel in the concrete structure. Matsuoka et al., in patent U.S. Pat. No. 4,861,453 entitled "Corrosion detecting probe for steel buried in concrete," describe a probe comprising of an electrode assembly placed within a container filled with an electrolyte solution. The container, made of an electrically insulating material includes an open end which is placed in close contact with a concrete surface under which is buried steel members. Said electrode assembly is attached via terminals to a electrochemical measuring apparatus for evaluating the corrosive state of the underlying steel members. Schiessi, in patent U.S. Pat. No. 5,015,355 entitled "Corrosion measuring cell," describes a cell comprising a corrosion-resistant cathode electrode and ordinary steel anodes which are separated and embedded in a concrete section for which the degree of corrosion in the underlying steel reinforcement members is desired. The cathode and anodes are electrically connected via a current measuring device. Feliu et al., in patent U.S. Pat. No. 5,259,944 entitled "Corrosion detecting probes for use with a corrosion-rate meter for electrochemically determining the corrosion rate of reinforced concrete structures," describe a probe comprising an electrode assembly which is positioned on a concrete surface containing metallic members. A corrosion-rate meter is coupled to the electrode assembly as well as the metal reinforcement members and wet spongy material is used electrolytic conduction. Bernhard, in patent U.S. Pat. No. 5,403,550 entitled "Electrode for determining the state of corrosion of metal reinforcement in concrete constructions," present an electrode comprising an insulated wire made of metal more electrochemically positive than the metal reinforcement members. This wire is embedded in the concrete and placed in electrolytic contact with the metal reinforcement members via moisture in the concrete. The insulation of the wire is either plastic or a material with equivalent electrical properties to that of concrete. A separate apparatus is used to measure the potential between the wire electrode and the metal reinforcement members.
None of the inventions described in these patents disclose a multilayer reference electrode having high stability for oxygen reduction and utilizing a catalytically active impregnated carbon electrode coated with an electrically conductive permeable ceramic coating for enhanced robustness and stability